1. Field of the Invention
The field of the invention is that of transition elements between electromagnetic waveguides.
In the field of microwave technology, waveguides are elements that provide for the guided transmission of an electromagnetic signal, for example between a source and a radiating element. The most widely used microwave transmission elements are the rectangular guide, the circular guide and the coaxial guide.
Transition elements are elements that are interposed simply between two guides of different types to obtain a change in transmission technology. Thus, there are transition elements that can be used to change from rectangular guide to coaxial guide technology, from rectangular guide to circular guide technology, from circular guide to coaxial guide technology, and vice versa.
The most frequently used transitions are those that enable changing from rectangular or circular guide technology to coaxial guide technology.
Circular guides are preferably used in certain frequency bands, because they have notable advantages. They are easier to make than rectangular waveguides, and their circular configuration enables them to be used as rotating joints (notably in the field of rotary antennas used for air and maritime surveillance) mechanically dissociating a fixed assembly from a movable assembly without creating any discontinuity in the guided propagation.
An object of the present invention is precisely the transitions between circular electromagnetic waveguides and coaxial electromagnetic waveguides.
2. Description of the Prior Art
In a known way, the passage from a circular guide to a coaxial guide is achieved of an internal conductor with a conical form as shown in FIG. 2, or more generally, with a form progressively radially extending and without transition step.
FIG. 2 shows a longitudinal section of a transition between a circular guide and a coaxial guide.
An electromagnetic wave gets propagated along a direction 24 in a circular guide 21 to which there is connected a transition 22 with a radius A having, at its center, a conical conductor 20. The conical conductor 20 constitutes an end of a circular conductor 23 with a radius B forming the central conductor of a coaxial guide 25. The transition 22 constitutes an end of a coaxial waveguide 25. The coaxial guide 25 is constituted by two conductors 23, 26 with outer radius A and inner radius B and a dielectric 27 enabling the internal conductor 23 to be placed coaxially within the external guide 26. The dielectric may either completely fill the section between the internal conductor 23 and the external guide 26 throughout the length on which the coaxial guide extends, or it may consist of thin round wafers of dielectric spaced out from one another and positioned evenly along the coaxial guide. Naturally, the dielectric chosen should not disturb the wave transmission that is carried out.
The gradual transition 22 is characterized by an angle .alpha.. Usually, the value of the angle .alpha. is between 7 and 10 degrees, depending on the passband and on the standing wave ratio (SWR) desired. The relationships between the SWR, the passband and the angle .alpha. are such that the angle .alpha. should be small if a high passband or a low SWR (little mismatching, major level of transferred power) is desired.
Thus, so that the transitions made do not excessively restrict the passband and do not prompt excessive reflections due to mismatching, it is necessary to choose a small angle .alpha., for a constant radius B of the central conductor, whence the relatively large transition length 22.
A large transition length is no negligible drawback, especially when no compromise can be accepted on the transmission characteristics. Thus, to preserve a suitable cut-off frequency and passband, it is not always possible to reduce the radius B of the central conductor 23 to diminish the length of the transition 22.
Besides, the longer the transition 22, the greater is the effect of its weight. This is a major drawback, notably when a transition 22 such as this has to form part of a device mounted on a satellite.
Another drawback of known transitions is that the end 28 of the conical part 20 of the central conductor 23 should absolutely be placed at the center of the circular waveguide 21 so as not to excite undesired modes, especially the TEM (transverse electrical magnetic) mode of the coaxial waveguide 25 which can get propagated irrespectively of the transmission frequency.